In the last forty years, a large number of oil additives and synthetic lubricants have been developed in order to improve the performance of or replace petroleum distillates as lubricants that reduce friction and wear between machinery parts. Additives such as oxidation inhibitors, rust inhibitors, antiwear agents and detergents have been used to improve the performance of petrolubricants or to extend their range of uses.
Oxidation inhibitors, for instance, are usually organic compounds containing sulfur, nitrogen, phosphorus or alkyl phenols which block the formation of hydroperoxides and so delay the appearance of sludge, varnish and organic acids in the oil, giving the oil extended life. Detergents act to prevent high-temperature deposit of oil-insoluble compounds on the lubricated parts by adsorbing to free particles and keeping them in solution.
Synthetic lubricants have been developed for environments which are not effectively or cheaply lubricated with conventional lubricants. Silicone oils, for example, have been found especially useful at high temperatures where many petrolubricants would burn off or decompose. On the other hand, synthetics have their own drawbacks. The silicone oils, for instance, often show poor lubricity in steel-to-steel rubbing environments, and have been most useful in gears or roller bearings, where lubricity is not as great a factor in lubricant performance.
In recent years, the diminishing availability of petroleum has led to an increased interest in designing lubricants containing lesser amounts of petroleum distillates and in developing synthetic lubricants with multi-purpose capabilities. Although this interest has sparked to creation of a number of new commercial lubricants, an all-purpose lubricant having high penetration and high lubricity while offering low migration and resistance to corrosion has not been found.